For many materials, including practically every man-made synthetic material, the mechanical behavior during processing as well as end product conditions is an important parameter that must be tightly specified and controlled. During the initial phases in the development of a new polymer or process, an understanding of the relationship between chemical structure and the physical properties of the process is of vital concern. Later on, in the process and quality control stages, factors such as mechanical strength, dimensional and thermal stability, and impact resistance are of utmost importance.
Virtually all synthetic materials in existence are viscoelastic, i.e., their behavior under mechanical stress lies somewhere between that of a pure viscous liquid and that of a perfectly elastic spring. Few materials behave like a perfect spring or a pure liquid. Rather, the mechanical behavior of these materials is generally time and/or temperature dependent and has led to such tests as creep, stress relaxation, tear, impact resistance, etc. One of the more important properties of materials sought is the materials' behavior under dynamic conditions. To explore this, a material's response to a cyclical stress as a function of temperature, time or frequency is determined. If a simple of a viscoelastic solid, for example is deformed and then released, a portion of the stored deformation energy will be returned at a rate which is a fundamental property of the material. That is, the sample goes into damped oscillation. A portion of the deformation energy is dissipated in other forms. The greater the dissipation, the faster the oscillation dies away. If the dissipated energy is restored the sample will vibrate at its natural (resonant) frequency. The resonant frequency is related to the modulus (stiffness) of the sample. Energy dissipation relates to such properties as impact resistance, brittleness, noise abatement, etc.
Because of their viscoelastic nature, the stress and strain in viscoelastic materials are not in phase, and, in fact, exhibit hysteresis. If a plot is made of this relationship, the area enclosed by the plot corresponds to the energy dissipated during each cycle of deformation of the material. In order to accurately describe this phenomenon, a complex modulus E= E' + jE" is often used to characterize the material where E is Young's modulus, E' is the real part and E" is the imaginary part. The real part E' of the modulus corresponds to the amount of energy that is stored in the strain and can be related to the spring constant, the complex part E" corresponds to the energy dissipation or damping and can be related to the damping coefficient used in second order differential equations to define vibrating systems.
Many dynamic mechanical analyzers have been developed over the years for measuring these properties. A dynamic mechanical analyzer is an instrument for measuring the modulus and mechanical damping of a material as a function of temperature (or time). Unfortunately, most of these known analyzers have a relatively limited dynamic range. This severely limits the type of samples (modulus) that can be studied.
One such instrument is known as the torsion pendulum in which an inertia member is attached to a sample of carefully shaped geometry. The mechanical system is set into torsional oscillations by the operator or by a driving pulse and the amplitude of the resulting free decaying oscillation is recorded. The frequency of oscillation can be related to the complex shear modulus by known formulas and damping can be related to the logarithmic decrement in amplitude by other known formulas. While simple in concept, the torsional pendulum usually requires complex manipulations, high operator skill, and at least one man-day to obtain any meaningful data therefrom.
Another known dynamic mechanical analyzer, the Rheovibron, exercises the sample into periodic longitudinal extensions by an electromechanical drive. The input displacement and output force (strain and stress) are measured by two strain gauges. When the amplitude of the two vector quantities are equal, their algebraic difference is approximately equal to the tangent of delta (the angle of the vector E) when the angle is small. Unfortunately, this instrument, whereas simple again in the theory, has a number of disadvantages. One is that for high damping values, errors as great as 50% can and do occur. Further very precise near optical alignment of the shafts coupling to the sample is required. Finally, the sample must be strained to near its yield point on the stress/strain curve. For many viscoelastic samples, this is a nearly impossible condition to fulfill.
A major improvement over these prior art instruments was made by Shilling with a dynamic mechanical analyzer. Shilling clamps a sample between the ends of two rigidly mounted tines. The bent thus formed is set into vibration at its resonant frequency, which is determined partially by the sample, and subjects the sample under test to a shear stress. This system is somewhat limited in the minimum frequencies over which it can operate by the stiffness of the tines. Furthermore, since the drive must be separated from the sample region which typically is held in an oven or other thermal enclosure, an excessive amount of power is required to drive the system. Finally, the unit is not dynamically balanced and hence is easily upset by vibration and spurious vibration modes.
Accordingly, it is an object of this invention to obviate many of the disadvantages of the prior art dynamic mechanical analyzers.
Another object of this inventin is to provide an improved dynamic mechanical analyzer which is capable of operating over a wide dynamic range.
An additional object of this invention is to provide an improved dynamic mechanical analyzer in which the sample under test contributes more to the analyzer's operation.